Carboxylic acidsEdexcel International A Level Chemistry: Subtopic test

1. 1
   Ethanoic acid (Mr 60) boils at 118 °C, whereas propan-1-ol (Mr 60) boils at 97 °C and propanal (Mr 58) boils at 49 °C. Ethanoic acid is fully miscible with water.

   (a)

   Which statement best explains why ethanoic acid has a higher boiling temperature than propan-1-ol?

   [1 mark]

   • AEthanoic acid has stronger covalent bonds within its molecules
   • BEthanoic acid molecules form more hydrogen bonds, as both the O–H group and the C=O oxygen take part
   • CEthanoic acid has a larger relative molecular mass
   • DEthanoic acid molecules are held together by ionic bonds

   (b)

   Which statement best explains why ethanoic acid is miscible with water?

   [1 mark]

   • AEthanoic acid molecules form ionic bonds with water molecules
   • BEthanoic acid has strong London forces with water
   • CEthanoic acid reacts completely with water to form carbon dioxide
   • DEthanoic acid forms hydrogen bonds with water through its O–H group and the oxygen atoms

   (c)

   Explain why the solubility of carboxylic acids in water decreases as the length of the carbon chain increases.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist prepares ethanoic acid by two different routes. In the first, ethanol is heated under reflux with an excess of acidified potassium dichromate(VI). In the second, ethanenitrile is heated under reflux with dilute hydrochloric acid.

   (a)

   Why is ethanol heated under reflux with an excess of acidified potassium dichromate(VI), rather than distilled, in the first route?

   [1 mark]

   • ASo that ethanal distils off as soon as it forms
   • BSo that ethanol is reduced to ethanoic acid
   • CSo that the ethanal first formed is kept in the flask and oxidised further to ethanoic acid
   • DSo that the dichromate(VI) is reduced to chromium(II)

   (b)

   Which compound is formed, as well as ethanoic acid, when ethanenitrile is heated under reflux with dilute hydrochloric acid?

   [1 mark]

   • AAmmonium chloride, NH₄Cl
   • BAmmonia, NH₃
   • CEthylamine, CH₃CH₂NH₂
   • DHydrogen, H₂

   (c)

   Write a balanced equation for the hydrolysis of ethanenitrile by dilute hydrochloric acid.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Propanoic acid is reacted separately with (i) aqueous sodium carbonate, (ii) phosphorus(V) chloride, (iii) ethanol with a few drops of concentrated sulfuric acid, heated under reflux, and (iv) lithium tetrahydridoaluminate(III) in dry ether.

   (a)

   Describe what is seen in reaction (i) and name the organic product of reaction (ii). Write a balanced equation for reaction (i).

   [3 marks]

   (b)

   Write equations for reactions (iii) and (iv), name the organic product of (iii) and state the role of the concentrated sulfuric acid in (iii).

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An organic chemist converts propan-1-ol into propyl propanoate in two stages. In stage 1, propan-1-ol is oxidised to propanoic acid. In stage 2, 7.40 g of propanoic acid is heated with excess propan-1-ol and a few drops of concentrated sulfuric acid, and 8.70 g of pure propyl propanoate is obtained.

   (a)

   Describe how stage 1 is carried out, including the apparatus, the reagents and the observation, and explain why these conditions are chosen.

   [6 marks]

   (b)

   Write an equation for stage 2, state the conditions needed, and calculate the percentage yield of propyl propanoate. Explain why the yield is below 100%.

   [6 marks]

   Total for question 4: 12 marks

End of questions